Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Plastics technology. Часть 1. Учебное пособие.pdf
21
At high reaction temperatures (e.g. 200°C) much higher pressures
are required to obtain a given concentration or density of monomer than at
temperatures of say 25°C and it might appear that better results would be
obtained at lower reaction temperatures. This is in fact the case where a
sufficiently active initiator is employed. This approach has an additional
virtue in that side reactions leading to branching can be suppressed. For a
given system the higher the temperature the faster the reaction and the
lower the molecular weight.
By varying temperature, pressure, initiator type and composition,
by incorporating chain transfer agents and by injecting the initiator into the
reaction mixture at various points in the reactor it is possible to vary
independently of each other polymer characteristics such as branching,
molecular weight and molecular weight distribution over a wide range
without needing unduly long reaction times. In spite of the flexibility,
however, most high-pressure polymers are of the lower density range for
polyethylene (0.915-0.94 g/cm3) and usually also of the lower range of
molecular weights.
Ziegler processes
These processes are largely due to the work of Ziegler and
coworkers. The type of polymerization involved is sometimes referred to as
co-ordination polymerization since the mechanism involves a catalystmono mer c o -ordination complex or some other directing force that controls
the way in which the monomer approaches the growing chain. The coordination catalysts are generally formed by the interaction of the alkyls of
Groups I–III metals with halides and other derivatives of transition metals
in Groups IV–VIII of the Periodic Table. In a typical process the catalyst is
prepared from titanium tetrachloride and aluminium triethyl or some related
material.
In a typical process ethylene is fed under low pressure into the
reactor which contains liquid hydrocarbon to act as diluent. The catalyst
complex may be first prepared and fed into the vessel or may be prepared
in situ by feeding the components directly into the main reactor. Reaction is
carried out at some temperatures below 100°C (typically 70°C) in the
absence of oxygen and water, both of which reduce the effectiveness of the
catalyst. The catalyst remains suspended and the polymer, as it is formed,
becomes precipitated from the solution and a slurry is formed which
progressively thickens as the reaction proceeds. Before the slurry viscosity
becomes high enough to interfere seriously with removing the heat of
22
reaction, the reactants are discharged into a catalyst decomposition vessel.
Here the catalyst is destroyed by the action of ethanol, water or caustic
alkali. In order to reduce the amount of metallic catalyst fragments to the
lowest possible values, the processes of catalyst decomposition, and
subsequent purification are al l important, particularly where the polymer is
intended for use in high-frequency electrical insulation.
The Ziegler polymers are intermediate in density (about
0.945g/cm3) between the high-pressure polyethylenes and those produced
by the Phillips and Standard Oil (Indiana) processes. A range of molecular
weights may be obtained by varying the Al-Ti ratio in the catalyst, by
introducing hydrogen as a chain transfer agent and by varying the reaction
temperature.
Over the years, considerable improvements and extensions of the
Ziegler process have taken place. One such was the advent of metallocene
single-site catalyst technology in the late 1980s. In these systems the olefin
only reacts at a single site on the catalyst molecules and gives greater
control over the process. One effect is the tendency to narrower molecular
weight distributions. In a further extension of this process Dow in 1993
announced what they refer to as constrained geometry homogeneous
catalysts. The catalyst is based on Group IV transition metals such as
titanium, covalently bonded to a monocyclopentadiene group bridged with
a heteroatom such as nitrogen. The catalyst is activated by strong Lewis
acid systems. These systems are being promoted particularly for use with
linear low-density polyethylene.
The Phillips process
In this process ethylene, dissolved in a liquid hydrocarbon such as
cyclohexane, is polymerized by a supported metal oxide catalyst at about
130-160°C and at about 1.4-3.5 MPa pressure. The solvent serves to
dissolve polymer as it is formed and as a heat transfer medium but is
otherwise inert.
The preferred catalyst is one which contains 5% of chromium
oxides, mainly CrO3, on a finely divided silica-alumina catalyst (75-90%
silica) which has been activated by heating to about 250°C. After reaction
the mixture is passed to a gas-liquid separator where the ethylene is flashed
off, catalyst is then removed from the liquid product of the separator and
the polymer separated from the solvent by either flashing off the solvent or
precipitating the polymer by cooling.
Polymers ranging in melt flow index (an inverse measure of
23
molecular weight) from l ess than 0.1 to greater than 600 can be obtained by
this process but commer cial products have a melt flow index of only 0.2-5
and have the highest density of any commercial polyethylene (~0.96g/cm3).
It is found that the molecular weight of the product is critically
dependent on temperature and in a typical process there is 40-fold increase
in melt flow index, and a corresponding decrease in molecular weight, in
raising the polymerization temperature from 140°C to just over 170°C.
Above 2.8 MPa the reaction pressure has little effect on either molecular
weight or polymer yield but at lower pressures there is a marked decrease
in yield and a measurable decrease in molecular weight. The catalyst
activation temperature also has an effect on both yield and molecular
weight. The higher the activation temperature the higher the yield and the
lower the molecular weight. A number of materials including oxygen,
acetylene, nitrogen and chlorine are catalyst poisons and very pure
reactants must be employed.
In a variation of the process polymerization is carried out at about
90-100°C, which is below the crystalline melting point and at which the
polymer has a low solubility in the solvent. The polymer is therefore
formed and removed as a slurry of granules each formed around individual
catalyst particles. High conversion rates are necessary to reduce the level of
contamination of the product with catalyst and in addition there are
problems of polymer accumulation on reactor surfaces. Because of the
lower polymerization temperatures, polymers of higher molecular mass
may be prepared.
Standard Oil Company (Indiana) process
This process has many similarities to the Phillips process and is
based on the use of a supported transition metal oxide in combination with
a promoter. Reaction temperatures are of the order of 230-270°C and
pressures are 40-80 atm. Molybdenum oxide is a catalyst that figures in the
literature and promoters include sodium and calcium as either metals or as
hydrides. The reaction is carried out in a hydrocarbon solvent.
The products of the process have a density of about 0.96 g/cm3,
similar to the Phillips polymers. Another similarity between the processes
is the marked effect of temperature on average molecular weight. The
process is worked by the Furukawa Company of Japan and the product
marketed as Staflen.
24
Processes for making linear low-density polyethylene and
metallocene polyethylene
Over the years many methods have been developed in order to
produce polyethylene with short chain branches but no long chain branches.
Amongst the earliest of these were a process operated by Du Pont Canada
and another developed by Phillips, both in the late 1950s. More recently
Union Carbide has developed a gas phase process. Gaseous monomers and
a catalyst are fed to a fluid bed reactor at pressures of 0.7-2.1 MPa at
temperatures of 100°C and below. The short branches are produced by
including small amounts of propene, but-1-ene, hex-1-ene or oct-1-ene into
the monomer feed. Somewhat similar products are produced by Dow using
a liquid phase process, thought to be based on a Ziegler-type catalyst
system and again using higher alkenes to introduce branching.
Figure 6 – Union Carbide gas phase process
for the production of polyethylene
During the late 1970s, Union Carbide developed a low-pressure
polymerization process (Unipol process) capable of producing polyethylene
25
in the gas phase that required no solvents. The process employed a
chromium based catalyst. In this process (Figure 6) ethylene gas and solid
catalysts are fed continuously to a fluidized bed reactor. The fluidized
material is polyethylene powder which is produced as a result of
polymerization of the ethylene on the catalyst. The ethylene, which is
recycled, supplies monomer for the reaction, fluidizes the solid, and serves
as a heat-removal medium. The reaction is exothermic and is normally run
at temperatures 25-50°C below the softening temperatures of the
polyethylene powder in the bed. This operation requires very good heat
transfer to avoid hot spots and means that the gas distribution and
fluidization must be uniform.
The keys to the process are active catalysts. These are special
organochromium compounds on particular supports. The catalysts yield up
to about 106 kg of polymer per kilogram of metallic chromium. Branching
is controlled by the use of comonomers like propylene or 1-butene, and
hydrogen is used as a chain transfer agent. The catalyst is so efficient that
its concentration in the final product is negligible. The absence of a solvent
and a catalyst removal step makes the process less expensive. The products
marketed as linear LLDPE can be considered as linear polyethylene having
a significant number of branches (pendant alkyl groups). The linearity
imparts strength, the branches impart toughness.
LLDPE materials are now available in a range of densities from
around 0.900 g/cm3 for VLDPE materials to 0.935 g/cm3 for ethyleneoctene copolymers. The bulk of materials are of density approx. 0.920
g/cm3 using butene in particular as the comonomer.
In recent years the market for LLDPE has increased substantially
and is now more than half the total for LDPE and for HDPE.
During the late 1990s several systems were developed where the
new catalysts could be employed in existing polymerization processes for
producing LLDPE-type polymers. These include high pressure autoclave
and solution processes as well as gas phase processes. At the present time it
remains to be seen what methods will become predominant.
Structure and properties of polyethylene
The polymer is essentially a long chain aliphatic hydrocarbon of
the type and would thus be thermoplastic.
–CH2–CH2–CH2–CH2–
Polyethylene, in essence a high molecular weight alkane (paraffin),
would be expected to have a good resistance to chemical attack and this is
found to be the case.
26
The polymer, in the absence of impurities, would also be expected
to be an excellent high-frequency insulator because of its non-polar nature.
Possibiliti es of branching in high-pressure polyethylene were first
expressed when investigation using infrared spectroscopy indicated that
there were about 20-30 methyl groups per 1000 carbon atoms. Therefore in
a polymer molecule of molecular weight 26000 there would be about 40-60
methyl groups, which is of course far in excess of the one or two methyl
groups to be expected from normal chain ends. More refined studies have
indicated that the methyl groups are probably part of ethyl and butyl
groups.
Short chain branching is negligible with Ziegler and Phillips
homopolymers although it is possible to introduce deliberately up to about
seven ethyl side chains per 1000 carbon atoms in the Ziegler polymers.
One effect of long chain branches is on flow properties.
Unbranched polymers have higher melt viscosities than long-branched
polymers of similar weight average molecular weight. This would be
expected since the long-branched molecules would be more compact and
be expected to entangle less with other molecules.
The more recently developed so-called linear low-density
polyethylene are virtually free of long chain branches but do contain short
side chains as a result of copolymerizing ethylene with a smaller amount of
a higher alkene such as oct-1-ene. Such branching interferes with the ability
of the polymer to crystallize as with the older low-density polymers and
like them have low densities. The word linear in this case is used to imply
the absence of long chain branch es.
Differences in molecular weight will also give rise to differences in
properties. The higher the molecular weight, the greater the number of
points of attraction and entanglement between molecules. Whereas
differences in short chain branching and hence degree of crystallinity
largely affect properties characterized by small solid displacement,
molecular weight differences will affect properties that involve large
deformations such as ultimate tensile strength, elongation at break, melt
viscosity and low-temperature brittle point. There is also an improvement
in resistance to environmental stress cracking with increase in molecular
weight.
Before the advent of Ziegler and Phillips polymers it was common
practice to characterize the molecular weight for technological purposes by
the melt flow index (MFI). From measurements of MFI various workers
27
have calculated the apparent viscosity of the polymer and correlated these
figures with both number average and weight average molecular weight.
Suffice it to say that the higher the melt flow index, the lower the molecular
weight.
Commercial polyethylene also vary in their molecular weight
distribution (MWD). Much of recent development in polymerization
technology has been devoted to establishing control of the MWD of
LLDPE polymers. With such polymers, narrowing the MWD confers
higher toughness, greater clarity, lower heat seal initiation temperatures
and, where this is important, higher cross-link efficiency. As with LDPE
there is lower melt shear sensitivity and poorer melt strength. Catalyst
systems have been used which result in polymers with a bimodal (doublepeaked) molecular weight distribution in an attempt to improve flow
properties, whilst another approach combines the use of polymers with
narrow molecular weight distribution but with a broad side-chain length
distribution.
A number of comonomers have been used in conjunction with
ethylene. Such comonomers are either hydrocarbons such as propyl ene or
but-1-ene non-hydrocarbons such as vinyl acetate. Small amounts of a
second alkene are sometimes used to produce a controlled degree of short
chain branching and some retardation in the growth of large crystal
structures. The use of hydrocarbon comonomers such as oct-1-ene became
very common with the development of LLDPE and this approach is also
being used with metallocene polyethylene. Properties of metallocene
polyethylene such as low density, lower melt temperatures, clarity and heat
sealability would be expected to be more related to the presence of
copolymers than the narrow molecular weight distribution (which has a
more significant effect on toughness and melt flow properties). Small
amounts of vinyl acetate also impede crystallization and, as with the alkene
copolymers, substantial amounts of the second comonomer lead to rubbery
materials.
The final variable to be mentioned here is the presence of
impurities. These may be metallic fragments residual from Ziegler-type
processes or they can be trace materials incorporated into the polymer
chain. Such impurities as catalyst fragments and carbonyl groups
incorporated into the chain can have a serious adverse influence on the
power factor of the polymer, whilst in other instances impurities can have
an effect on aging behaviour.
Polyethylene is a wax-like thermoplastic softening at about 80-
28
130°C with a density less than that of water. It is tough but has moderate
tensile strength, is an excellent electrical insulator and has very good
chemical resistance. In the mass it is translucent or opaque but thin films
may be transparent.
Mechanical properties
The mechanical properties are very dependent on the molecular
weight and on the degree of branching of the polymer. As with other
polymers these properties are also dependent on the rate of testing, the
temperature of test, the method of specimen preparation, the size and shape
of the specimen and, to only a small degree with polyethylene, the
conditioning of samples before testing. It should also be remembered that
polymers of different density but with the same melt flow index do not
have the same molecular weight.
The elongation at break of polyethylene is strongly dependent on
density, the more highly crystalline high-density materials being less
ductile.
Thermal properties
Tough at room temperature, the polymers become brittle on cooling
but some specimens do not appear to become brittle until temperatures as
low as -70°C have been reached. In general the higher the molecular weight
and the more the branching the lower the brittle point. Measured brittle
points also depend on the method of sample preparation, thus indicating
that the polymer is notch sensitive, i.e. sensitive to surface imperfections.
The specific heat of polyethylene is higher than for most
thermoplastics and is strongly dependent on temperature. Low-density
materials have a value of about 2.3 J/g at room temperature and a value of
2.9 J/g at 120-140°C.
Flow properties of polyethylene have been widely studied. Because
of the wide range of average molecular weights amongst commercial
polymers the viscosities vary widely. The most commonly used materials,
however, have viscosities lower than for unplasticized PVC and
poly(methyl methacrylate) and higher than for the nylons.
It is interesting to note that so-called linear low-density
polyethylene are said to be less pseudoplastic than conventional lowdensity polyethylene. Thus on comparing the two materials at the same
melt flow index the “linear” polymer will be found to be more viscous at
the higher shear rates usually encountered during processing.
29
Melt elasticity is of considerable importance in understanding
CH
2
CH
2
CH
2
CH
2
CH
2
+
2Cl
2
+ SO
2
CH
CH
2
CH
2
CH
2
CH
Cl
SO
2
Cl
much of the behaviour of polyethylene when processing by film extrusion
techniques and when blow moulding. The complex relationships observed
experimentally here have been summarized by the author elsewhere.
Chemical properties
The chemical resistance of polyethylene is, to a large measure, that
expected of an alkane. It is not chemi cally attacked by non-oxidising acids,
alkalis and many aqueous solutions. Nitric acid oxidises the polymer,
leading to a rise in power factor and to a deterioration in mechanical
properties. As with the simple alkanes, halogens combine with the
hydrocarbon by means of substitution mechanisms.
When polyethylene is chlorinated in the presence of sulphur
dioxide, sulphonyl chloride as well as chlorine groups may be incorporated
into the polymer. This reaction is used to produce a useful elastomer
(Hypalon).
Oxidation of polyethylene which leads to structural changes can
occur to a measurable extent at temperatures as low as 50°C. Under the
influence of ultraviolet light the reaction can occur at room temperature.
The oxidation reactions can occur during processing and may initially
cause a reduction in melt viscosity. Further oxidation can cause
discolouration and streaking and in the case of polymers rolled for 1-2
hours on a two-roll mill at about 150°C the product becomes incapable of
flow.
Since polyethylene is a crystalline hydrocarbon polymer incapable
of specific interaction and with a melting point of about 100°C, there are no
solvents at room temperature. Low-density polymers will dissolve in
benzene at about 60°C but the more crystalline high-density polymers only
dissolve at temperatures some 20-30°C higher. Materials of similar
solubility parameter and low molecular weight will, however, cause
swelling, the more so in low-density polymers.
Low-density polyethylene has a gas permeability in the range
normally expected with rubbery materials. This is because in the
amorphous zones the free volume and segmental movements facilitate the
passage of small molecules. Polymers of the Phillips type (density 0.96
g/cm3) have a permeability of about one-fifth that of the low-density
materials.
30
Exposure of polyethylene to ultraviolet light causes eventual
embrittlement of the polymer. This is believed to be due to the absorption
of energy by carbonyl groups introduced into the chain during
polymerization and/or processing.
When polyethylene is subjected to high-energy irradiation, gases
such as hydrogen and some lower hydrocarbons are evolved, there is an
increase in unsaturation and, most important, cross-linking occurs by the
formation of С–С bonds between molecules. The formation of cross-link
points interferes with crystallization and progressive radiation will
eventually yield an amorphous but cross-linked polymer. Extensive
exposure may lead to colour formation and in the presence of air surface
oxidation will occur. Oxygen will cause polymer degradation during
irradiation and this offsets the effects of cross-linking. Long exposure to
low radiation doses on thin film in the presence of oxygen may lead to
serious degradation but with short exposure, high radiation doses and
thicker specimens the degradation effects become less significant. Since
cross-linking is accompanied by a loss of crystallization, irradiation does
not necessarily mean an increased tensile strength at room temperature.
However, at temperatures about 130°C irradiated polymer still has some
strength (it is quite rubbery), whereas the untreated material will have
negligible tenacity. It is found that incorporation of carbon black into
polyethylene which is subsequently irradiated can give substantial
reinforcement whereas corresponding quantities in the untreated product
lead to brittleness.
Electrical properties
The insulating properties of polyethylene compare favourably with
those of any other dielectric material. As it is a non-polar material,
properties such as power factor and dielectric constant are almost
independent of temperature and frequency. Dielectric constant is linearly
dependent on density and a reduction of density on heating leads to a small
reduction in dielectric constant.
Oxidation of polyethylene with the formation of carbonyl groups
can lead to a serious increase in power factor. Antioxidants are
incorporated into compounds for electrical applications in order to reduce
the effect.